1. Technical Field
The invention relates generally to low thermal expansion glass for extreme ultraviolet lithography (EUVL) applications.
2. Description of Related Art
Optical lithography systems have a system resolution, RES, that is a function of three parameters: process-related factor k1, wavelength of exposure light λ, and numerical aperture NA. Equation (1) below shows the relationship between RES, k1, λ, and NA.
                    RES        =                              k            1                    ⁢                      λ            NA                                              (        1        )            
The value of RES determines the smallest feature that can be printed by the system. The smaller the value of RES, the smaller the feature that can be printed. RES is inversely proportional to NA and directly proportional to k1 and λ. Therefore, a combination of decreasing k1 and λ and increasing NA can be used to decrease the value of RES. However, k1, λ, and NA cannot be changed infinitely or haphazardly because of process and material limitations and because the choice of λ and NA also affects the depth of focus (DOF), as shown in Equation (2).
                    DOF        =                              ±                          k              2                                ⁢                      λ                                          (                NA                )                            2                                                          (        2        )            
In Equation (2), k2 is a process-related factor. In general, a large DOF is desired, which would require a combination of increasing k2 and λ and decreasing NA—this is opposite to the strategy for decreasing RES.
Thus far, exposure wavelength λ has offered the most opportunities for change, with current lithography systems having progressed from 248 nm to 193 nm to 157 nm. At an exposure wavelength of 13 nm, EUVL is a giant leap forward from the current lithographic systems and offers a higher resolution and larger depth of focus than possible with current lithographic systems. EUVL tools are geared towards printing of feature sizes below 100-nm. However, commercialization of EUVL tools has not been easy. For instance, extreme ultraviolet (EUV) radiation is readily absorbed by virtually all known materials, which makes it impossible to adapt the refractive optics used in current lithography systems for EUVL systems. Reflective optics and masks have had to be developed for EUVL systems. These reflective optics and masks typically include reflective multilayer (ML) coatings on a substrate. A reflective multilayer consists of alternating layers of high-reflectance and low-reflectance materials, typically alternating layers of Mo and Si or Mo and Be.
Substrate materials for EUV reflective optics and masks are required to meet stringent requirements with respect to coefficient of thermal expansion (CTE) and surface roughness since any expansion or waviness in these materials during printing can distort the printing of features. For reflective imaging optics and masks especially, it is important that the substrate has a near-zero CTE at the application temperature. Glass or glass-ceramic with a low CTE is typically used as the substrate material. ZERODUR® glass-ceramic, made by Schott AG, and Ultra-Low Expansion (ULE®) glass, made by Corning Incorporated, have been identified as substrate materials for EUVL applications. ULE® glass is a titania-silicate glass with a titania (TiO2) content in a range from 5 to 10 wt %. Code 7972 ULE® glass has a mean CTE of 0±30 ppb/° C. at 5° C. to 35° C. ULE® glass is also highly polishable. U.S. Patent Publication Application No. 2008/0132150 A1 (Arserio et al.) describes a method for polishing ULE glass to a high-spatial frequency roughness of less than 0.20 nm rms. Typically, a high-spatial frequency roughness in a range from 0.005 to 0.30 nm rms is desired for EUVL applications.
CTE changes with temperature. Zero-CTE crossover of a material is the temperature at which the CTE of the material is 0 ppb/° C. Currently, the EUVL community wishes for a glass material having 0±5 ppb/° C. at 20° C. Code 7972 ULE® glass meets this criterion with a zero-CTE crossover at 20° C. Code 7972 ULE® glass also has a stable thermal expansion in the temperature range of approximately 0° C. to 40° C., which works well for the current generation of EUVL applications. With the next generation EUVL applications of higher power energy sources and increased thermal gradients, the specifications for EUV material will become more stringent. For these next generation EUVL applications, a low thermal expansion glass having a stable thermal expansion over a wider temperature range than possible with the current ULE® glass and that can be polished to meet surface roughness requirements is desired.